A radio based station eNB supporting LTE (Long Term Evolution), and a mobile station UE have a PHY (physical) layer, a MAC (Media Access Control) layer, a RLC (Radio Link Control) layer, a PDCP (Packet Data Convergence Protocol) layer, and a RRC (Radio Resource Control) layer as illustrated in FIG. 6.
The PDCP layer is configured to perform ciphering processing, tamper detection processing, and header compression processing.
Here, the ciphering processing and the tamper detection processing use a COUNT value. As illustrated in FIG. 7, a COUNT value includes an HFN (Hyper Frame Number) and a PDCP-SN (Sequence Number).
The PDCP-SN includes 12 bits or 7 bits and is configured to be incremented every time the PDCP layer sends a packet to the RLC.
In addition, the HFN includes 20 bits or 25 bits and is configured to be increment every time the PDCP-SN goes once through all the numbers.
When COUNT values are not synchronized between a PDCP layer on a transmission side and a PDCP layer on a reception side, deciphering processing cannot be correctly performed in the PDCP layer on the reception side.
Specifically, the PDCP layer on the transmission side is configured to perform ciphering processing (header compression processing and tamper detection processing) on a packet (PDCP-SDU) received from a RRC layer by using a COUNT value, and send the RLC layer the resultant received packet as PDCP-PDU with the PDCP-SN added to the header thereof.
On the other hand, the PDCP layer on the reception side is configured to manage a reception window and discard a received packet if the PDCP-SN added to the packet (PDCP-PDU) is PDCP-SN outside the reception window, as illustrated in FIG. 8.
Here, the PDCP layer on the reception side is configured to, if the PDCP-SN added to the received packet (PDCP-PDU) is PDCP-SN inside the reception window, estimate HFN to be used for deciphering the packet from a current reception state, send the packet (PDCP-SDU) deciphered using the estimated HFN to an upper layer, and update the reception window.
As illustrated in FIG. 9 and FIG. 10, when reconnection procedure is completed, the PDCP layer on the transmission side starts transmitting all the packets whose acknowledgements have not been received in the RLC layer.
The PDCP layer on the reception side sends all the packets sendable to the upper layer even in an “out-of-sequence” state, and again tries to perform reception inside the reception window after the reconnection procedure is completed.
Also, after the reconnection procedure is completed, the PDCP layer on the reception side can report a reception status of “PDCP-SN” in “PDCP status report”, whereas the PDCP layer on the transmission side can also cancel transmitting of packets reported as received.
As illustrated in FIGS. 11 and 12, however, the following problem may occur. Specifically, a radio base station S-eNB transfers PDCP-SDU whose RLC-ACK is not received to a radio base station T-eNB. If a large amount of PDCP-PDUs are multiplexed on an RLC-PDU, for example, “HFN mismatch” may occur between the radio base station T-eNB and a mobile station UE, so that the reception side fails in the deciphering processing, and cannot extract packets in the normal state.
In particular, if a large amount of PDCP-PDUs are multiplexed on one RLC-PDU, a possibility of occurrence of “HFN mismatch” is high, due to transmission of a large number of PDCP-PDUs for which the RLC-ACK has not been received.
For example, the possibility of occurrence of “HFN mismatch” becomes high when a transmission rate is high, or when a large amount of packets whose size of PDCP-SDU is extremely small are generated.
For this reason, to avoid the occurrence of “HFN mismatch,” proposed is to extend a PDCP-SN length, in other words, to adopt “extended PDCP-SN (see, FIG. 13(b))” which is a sequence number longer than “conventional PDCP-SN (see, FIG. 13(a)),” (Non-patent document 3).
Use of “extended PDCP-SN” makes the PDCP-SN less likely to go once through all the numbers, and thus makes “HFN mismatch” less likely to occur.
Here, a radio base station eNB is configured to instruct the use of “extended PDCP-SN” to a mobile station UE through RRC signaling.